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Full text of "Mathematical And Physical Papers - Iii"

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ON  THE  CHANGE  OF  REFKANG1BILITY  OF  LIGHT.
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an alcoholic solution. This medium was sensitive enough to exhibit a pretty copious dispersive reflexion of a pale greenish yellow light. Its sensibility was more confined than usual to the rays of very high refrangibility.
120.    Among  petals,  the   most   remarkable   which   I   have observed   are those  of the purple  groundsel  (Senecio  elegans). These petals disperse a red light, more copious  than is usual among  petals.     If a  petal  be  placed  behind  a  slit,  and the transmitted light be analysed, it is Tound to exhibit three remarkable bands of absorption, much resembling those  of blue glass, but closer together, and beginning later in the spectrum, the first appearing about the place of the orange.    These bands are still better seen in a solution of the colouring matter in weak alcohol.    On examining this medium by the third method, with a lens of shorter focus than usual, and looking down from above, the places of the absorption bands were indicated by tooth-shaped interruptions in the beam of light reflected from motes.    The points of these teeth were occupied by red dispersed light, which did not appear in the intervening beams of light reflected from motes, from whence it appears that there is the same sort of connexion between the absorption and dispersion of this medium as was noticed in Art. 59, in the case of solutions of chlorophyll and its modifications.
121.    A  collection  of sea-weeds  appeared all more or less sensitive, most of them highly so.    All, or almost all, except the white ones, exhibited in the derived spectrum the peculiar red band indicative of chlorophyll and its modifications.    The transmitted light also exhibited  more or less the absorption bands due to this substance, which was likewise, in the specimens tried, extracted   by   alcohol.     But  the  most  remarkable   example  of sensibility  found  in  sea-weeds  occurs  in  the  case  of the  red colouring matter contained in orangy red, red, pink, and purple sea-weeds.    To  judge   by  its  optical  properties,  this  colouring matter appears to be the same in all cases, but to be mixed in different proportions with chlorophyll, or some modification of it, and probably other colouring matters, thus giving  rise  to  the various  tints   seen   in   such   sea-weeds.    The  derived   spectrum exhibited by sea-weeds of this kind  consists mainly of a band of unusual brightness, containing some red, followed by orange
S. ill.                                                                     22